South African officials say poachers killed 21 rhinos in January, despite new efforts to crack down on the practice.

National Parks chief David Mabunda says the government is determined to stop the poaching, which killed 333 rhinos in 2010.

In a statement Monday, authorities said park rangers have killed five suspected poachers in skirmishes since the start of the year. They have also arrested 31 suspects.

Government officials and conservation groups say sophisticated criminal networks are behind a rise in rhino poaching. They say poachers use helicopters, night-vision equipment, silencers and animal tranquilizers to kill the animals and avoid being caught.

Rhino horns are in high demand on the black market. In some Asian countries, the horns are seen as having mysterious powers and are used as aphrodisiacs or healing agents.

South Africa is home to about 21,000 rhinos, more than any other country in the world.

The government launched a wildlife crime investigation unit in October.